bug: file watcher native callback loses ALS context

[kitlangton]

Bug

The @parcel/watcher native C++ callback fires outside the Instance AsyncLocalStorage context. Bus.publish needs Instance.directory to resolve per-instance subscribers and tag GlobalBus events, so it silently throws Context.NotFound — no log, no crash, events just vanish.

The git HEAD watcher is always active (no experimental flag), so this affects all users.

Impact

• TUI branch indicator never live-updates when switching branches externally (git checkout in another terminal)
• Vcs.Event.BranchUpdated never fires from the native watcher path
• Any Bus.subscribe(FileWatcher.Event.Updated, ...) listener silently receives nothing from native file changes

(Tool-initiated events from edit.ts, write.ts, apply_patch.ts still work because they call Bus.publish from within Instance.provide.)

Repro

This PR adds a failing test that:

1. Boots an instance with the file watcher
2. Listens on GlobalBus for watcher events
3. Triggers a git checkout -b to change .git/HEAD
4. Times out after 5s because the event never arrives

bun run test test/file/watcher-als-bug.test.ts

Fix

Addressed in #17601 via Instance.bind(fn) — captures the ALS context at subscription time and restores it synchronously in the native callback.